Abstract: During the Earth's history, the chemical composition of the atmosphere has evolved as a result of natural fluctuations in the climate system. Since the agricultural and industrial revolutions, the atmospheric abundance of several radioactively and/or chemically active trace gases (CO2, CH4, N2O, chlorofluorocarbons, etc.) Has increased dramatically, leading to potential greenhouse warming. Over the last decades, stratospheric ozone has been depleted, especially at mid- and high latitudes, leading to significant increase in the UV-B radiation level. Changes in tropospheric ozone have also been reported, while the oxidizing capacity of the atmosphere seems to have remained relatively unchanged. The lecture will review these observed changes, and analyze the processes involved, using global chemical-transport models, developed to diagnose observations and to predict the future state of the atmosphere. Special attention will be given to the effect on stratospheric ozone of the 11-year solar cycle, as well as of volcanic eruptions (e.g., Mt. Pinatubo). The impact of anthropogenic emissions (including aircraft effluents) on the composition of the global troposphere will also be discussed. Brochure